Ultrasound examination supporting CT or MRI in the evaluation of cervical lymphadenopathy in patients with irradiation-treated head and neck cancer

Abstract In this study, we determined the diagnostic performance of adding ultrasound (US) with/without fine‐needle aspiration cytology (FNAC) to computed tomography (CT)/magnetic resonance imaging (MRI) in evaluating neck lymphadenopathy (LAP) in patients with head and neck cancer treated with irradiation. We included 269 patients who had neck LAP after radiotherapy (RT) or concurrent chemoradiotherapy (CCRT) resulting from cancers of the head and neck region between October 2008 and September 2018. The diagnostic methods consisted of the following: 1) CT/MRI alone, 2) CT/MRI combined with a post-RT US predictive model, and 3) CT/MRI combined with US + FNAC. We compared their diagnostic performance using receiver operating characteristic (ROC) curves. In total, 141 (52%) malignant and 128 (48%) benign LAPs were observed. Regarding the diagnostic accuracy, the area under the ROC curves was highest for the combined CT/MRI and US + FNAC (0.965), followed by the combined CT/MRI and post-RT US predictive model (0.906) and CT/MRI alone (0.836). Our data suggest that the addition of a US examination to CT/MRI resulted in higher diagnostic performance than CT/MRI alone in terms of diagnosing recurrent or persistent nodal disease during the evaluation of LAP in patients with irradiation-treated head and neck cancer.


Introduction
Radiotherapy (RT) is the standard treatment for advanced stage head and neck cancer. However, following neck irradiation, the surveillance of nodal malignancy is challenging. RT may result in tissue fibrosis, increasing the difficulty of palpating an enlarged node [1,2]. The lymph node (LN) recurrence rate of head and neck cancer after RT ranges from <10 to 29% [2,3]. Magnetic resonance imaging (MRI) or computed tomography (CT) is typically applied to detect such recurrence [4], although ultrasound (US) has been gradually introduced for more precise detection [5,6]. The 2020 National Comprehensive Cancer Network (NCCN) guidelines report that US, CT, MRI, and positron emission tomography (PET)/CT have distinct advantages in the posttreatment follow-up of patients with locoregionally advanced head and neck cancer [7]. The follow-up intervals for CT, MRI, and PET/CT are clearly defined in the 2020 NCCN guidelines. However, US is typically used as an adjuvant imaging tool, and its role is less emphasized. In this study, we explored whether adding US with or without fine-needle aspiration cytology (FNAC) to CT/MRI improves diagnostic accuracy. Studies comparing the diagnostic rate of US and CT/MRI during the posttreatment evaluation of patients with head and neck cancer remain limited. Investigating patients with nasopharyngeal carcinoma (NPC), Toh et al. reported that the positive predictive value (PPV) of recurrent nodal metastasis was 93.8% for FNAC and 78.6% for CT during posttreatment followup [8]. In patients with head and neck cancer who had completed concurrent chemoradiotherapy (CCRT), Nishimura et al. observed that the sensitivity and specificity of diagnosing malignant LN were 52.9 and 74.2% for CT/MRI, 88.2 and 66.1% for US, and 71.4 and 95.6% for FNAC, respectively [9]. In our previous study, we developed a post-RT US predictive model for the prediction of recurrent or persistent nodal disease in irradiation-treated patients [10]. The model was 1.35 × (long axis) + 2.03 × (short axis) + 2.27 × (margin) + 1.48 × (echogenic hilum) + 3.7. If the score was equal to or greater than 7, an LN was regarded as malignant. This predictive model exhibited favorable sensitivity, PPV, negative predictive value (NPV), and accuracy (85, 82, 83, and 83%, respectively). In this study, we explored the effect of adding US with or without FNAC to CT/MRI in the assessment of recurrent or persistent lymphadenopathy (LAP) in patients with irradiationtreated head and neck cancer under a retrospective setting. Furthermore, we compared the diagnostic performance of CT/MRI alone, CT/MRI in combination with the post-RT US predictive model, and CT/MRI in combination with US + FNAC.

Inclusion and exclusion criteria
This retrospective study was performed at a tertiary medical center. The reporting of the study followed the Standards for Reporting Diagnostic accuracy studies (STARD) statement. Data from patients who received RT or CCRT for the treatment of cancers in the head and neck region between October 2008 and September 2018 were reviewed. Patients who had LAP, which was defined as the presence of one or many LN(s) detected through a palpation or imaging study, after neck irradiation were included in this study. We included both patients who had or did not have previous neck dissection. All patients received either CT or MRI together with US with or without FNAC approximately 2-3 months after RT/CCRT, followed by every 6 months or under suspicion of recurrence. The CT or MRI was performed with contrast under a 3 mm or 5 mm slice, respectively. Usually, we arranged MRI for the surveillance. If patients were intolerant to MRI examination due to claustrophobia, dyspnea, or not suitable for the prolonged supine position, we arranged CT for the evaluation. US was performed using the brightness and Doppler mode without contrast. Ultrasound-guided fine-needle aspiration (USgFNA) was performed in patients exhibiting suspicious US features during examination [11]. If a patient had one or multiple LNs with suspicion of malignancy, we chose the largest LN for USgFNA. Patients who were lost to follow-up after neck irradiation or did not undergo an imaging study were excluded ( Figure 1).
The final diagnoses were obtained according to either clinical diagnoses after multidisciplinary discussions or pathological diagnoses through means of core needle biopsy, excisional biopsy, or neck dissection. In clinical diagnosis, an LAP was regarded as benign when no size change was observed during the course of 12-month follow-up; an LAP was considered malignant when the disease was clearly observed during the imaging study, which was verified with an abnormal cytological report (malignancy, suspicion of malignancy, or atypical cells), or when an LAP was observed to be obviously enlarged during further image study.

Clinical characteristics and outcome assessment
We recorded information on the age, gender, duration between RT/CCRT and the imaging study, primary malignancy, CT or MRI imaging, US examination, and FNAC report from the medical records. The short axis, long axis, and short-to-long axis (S/L) ratio of neck LN were documented from the US images. Nishimura et al. evaluated the ability of CT/MRI, US, or FNAC to diagnose malignant LNs [9]. However, in clinical practice, FNAC is seldom performed without US. Thus, in this study, the single diagnostic examinations analyzed were CT/MRI, the post-RT US predictive model, and US + FNAC. The diagnosis of CT/MRI was determined by an experienced radiologist based on the patient's medical history. Following neck irradiation, an enhancement and expansion feature, such as an irregular margin, on an LN was regarded as a malignancy [12]. The post-RT US predictive model was that used in our earlier study [10]. If its score was ≥7, a node was considered to be malignant. The diagnosis of US + FNAC was mainly based on the cytological report, which was supported using the post-RT US predictive model. If the cytological report indicated the presence of malignancy, suspicion of malignancy, or atypical cells, this LN was regarded as malignant. If no cytological report was available, we used the post-RT US predictive model to determine LN malignancy or benignity.

Statistical analysis
A two-sample t-test was used for continuous variables, and the chi-squared or Fisher exact test was used for categorical variables. The odds ratio (OR) with a 95% confidence interval was reported. Other studies have only focused on the accuracy of single diagnostic examinations rather than that of combined methods. In our study, we evaluated the effect of adding a US examination to CT/MRI by assessing the following diagnostic methods: 1) CT/MRI alone, 2) CT/MRI combined with the post-RT US predictive model, and 3) CT/MRI combined with US + FNAC. According to the final diagnoses, we calculated the predicted probability for nodal malignancy using logistic regression. The diagnostic performance was also compared using receiver operating characteristic (ROC) curves and the area under the receiver operating characteristic curves (AUC). AUC differences by using paired-sample area differences under the ROC curves were also executed. Statistical significance was indicated if p < 0.05. Statistical analysis was conducted using SPSS software version 28 (IBM, Armonk, NY, USA).

Ethical considerations: This study was approved by the institutional ethical review board of Far Eastern Memorial
Hospital (No. 109140-E). The study did not influence the patients' treatment or outcome. All data were analyzed using a deidentified form; the data set is presented in the supplementary material (Table S1).

Results
A total of 269 patients who exhibited LAP following neck RT were included in our study ( We observed significant differences in age (p < 0.001), duration between RT/CCRT and imaging study (p = 0.04), short axis (p < 0.001), long axis (p < 0.001), and the S/L ratio (p < 0.001) but not gender ( Table 1). The diagnostic examinations for assessing neck LAP were compared according to the final diagnoses ( Table 2). We noted significant differences in distinguishing benign from malignant nodal disease in all three diagnostic exams (p < 0.001). The OR was highest in US + FNAC, followed by the CT/MRI report, and then the post-RT US predictive model (103.9, 30.1, and 28.4, respectively).
The predicted probability for malignancy for CT/MRI alone, the combined CT/MRI and post-RT US predictive model, and the combined CT/MRI and US + FNAC is presented in Table 3. For cases where the diagnostic exams Abbreviations: CT, computed tomography; FNAC, fine-needle aspiration cytology; LAP, lymphadenopathy; MRI, magnetic resonance imaging; RT/CCRT, radiotherapy/concurrent chemoradiotherapy; S/L, short-to-long axis; US, ultrasound. *Statistical significance, p < 0.05.

Discussion
Although the improved survival rate for the early detection of malignancy may result from lead time bias, regular surveillance for nodal recurrence or persistence is still crucial during the follow-up period after primary treatment for head and neck cancer. Several studies have demonstrated that early detection of malignancy benefits the survival rate [5,13,14]. However, for patients with head and neck cancer who have undergone neck irradiation, clinicians may face difficulty in the evaluation of nodal disease because of tissue fibrosis [1,2]. In this study, we determined that for detecting nodal recurrence or persistence, the combined CT/MRI and post-RT US predictive model had a higher AUC than that of CT/ MRI alone (0.906 vs 0.836; Figure 2). Moreover, we observed that the combined CT/MRI and US + FNAC resulted in an improved AUC (0.965). Consequently, the addition of a US examination to CT/MRI assisted in the early diagnosis of nodal malignancy in patients with irradiation-treated head and neck cancer. The combination of CT/MRI with US + FNAC remained the most accurate in terms of the diagnosis of nodal recurrence or persistence. At our institution, FNAC was performed simultaneously when suspicious echogenic findings for malignancy were noted during US studies. These findings included irregular margins, heterogeneous internal echogenicity, the presence of calcification, cystic architecture, absence of echogenic hilum, and a peripheral or mixed vascular pattern [11]. Our results revealed that the false-negative rate was significantly higher in patients who were diagnosed using CT/MRI alone than that of patients diagnosed using CT/MRI combined with US + FNAC (11% [29/269] vs 1% [3/269], p < 0.001) during the evaluation of neck LAP. Thus, we suggest not only performing a US examination during follow-up but also obtaining FNAC simultaneously when presented with suspicious ultrasonographic features in the assessment of neck LAP in patients with irradiation-treated head and neck cancer.   Table 2). This result may be attributable to post-RT LNs' tendency to have higher heterogeneity and lower radiodensity in contrast-enhanced CT imaging [15]. Furthermore, our earlier study indicated that the size of recurrent LNs tends to be smaller in patients with a history of RT than that in patients who have never undergone irradiation treatment [16]. Smaller LNs may not be easily detected using a 5 mm cut MRI or 3 mm cut CT. Therefore, a small and less enhanced malignant LN may be classified as benign in a CT/MRI report, generating a false-negative result (Figure 3). The US examination represents a high-resolution continuous imaging study for evaluation of the cervical node [17] and, with the assistance of the predictive model, could produce a higher NPV than CT/MRI.
Although we combined CT/MRI and US + FNAC to increase the diagnostic performance, three false negatives were observed (  [18]. A possible reason for this is the histological change of LNs after RT. Cancer cells within LNs might be isolated and unevenly distributed after RT treatment, leading to an increase in false-negative results [8,18]. To address this shortcoming and increase the diagnostic rate for NPC patients, the addition of plasma Epstein-Barr virus (EBV) DNA testing was proposed in one study [18], and the implementation of a combination PET examination during the follow-up period was suggested in another study [8]. Further study may evaluate the diagnostic ability for LAPs when combining the CT/MRI and US + FNAC with plasma EBV DNA or PET.

Limitations
This study has several limitations. First, unnoticed or unavoidable selection bias might have played a role owing to the retrospective study design. Second, this study is based on a convenience sample and we did not calculate the sample size initially. Third, not all our final diagnoses of nodal disease were obtained through pathological diagnosis. Some patients with obvious nodal disease or those that were unsuitable for neck dissection were diagnosed following FNAC and multidisciplinary discussions. Moreover, this study did not identify the most suitable interval at which US imaging studies should be performed. The frequency at which the US study must Oral cancer 13 (45%) 0 (0%) NPC 5 (17%) 3 (100%) Hypopharyngeal cancer 6 (21%) 0 (0%) Laryngeal cancer 2 (7%) 0 (0%) Unknown primary tumors of head and neck 2 (7%) 0 (0%) Nasal malignant melanoma 1 (3%) 0 (0%) Abbreviations: CT/MRI, computed tomography/magnetic resonance imaging; FNAC, fine needle aspiration cytology; RT/CCRT, radiotherapy/concurrent chemoradiotherapy; S/L, short-to-long axis; US, ultrasound. † Fisher exact test with corresponding OR and 95 CI. *Statistical significance, p < 0.05.
be conducted and the most cost-effective method requires further evaluation.

Conclusion
Surveillance of nodal recurrence or persistence is critical during post-RT follow-up in patients with head and neck cancer. Based on this study, CT/MRI combined with either the post-RT US predictive model or US + FNAC had stronger diagnostic performance than CT/MRI alone in assessing nodal malignancy in patients with LAPs treated with irradiation. Postirradiation recurrence or persistent LAPs tend to be more heterogeneous and smaller, which may lead to a lower accuracy rate in evaluations when employing CT/MRI alone. Besides, in this study, the US with CT/MRI was also performed when suspicion of recurrence. Therefore, the results cannot support regular screening over screening on indication. Although this was a retrospective study and the most suitable interval of US examination was not identified, we still recommend performing US studies alongside CT/MRI when suspicion of recurrence increases the early and precise diagnosis of nodal malignancy in patients with irradiation-treated head and neck cancer. Moreover, FNAC can be implemented simultaneously when suspicious ultrasonographic features are detected.